1. Field of the Invention
The present application is directed to a radar system in which a solid-state, long radar pulse transmitter is used with a short coded pulse radar processor and, more particularly, to a system in which a long pulse compression filter is provided in the receiver to convert the long pulse into the short pulse expected by the signal processor allowing retrofit of existing radar systems with the more reliable solid-state transmitters or to the design of a system in which hard limiting constant false alarm rate processing is desired.
2. Description of the Related Art
Many existing radars, such as the TPS-70 available from Westinghouse, use tube transmitters to generate, and a processor designed to process, relatively short coded radar pulses. A pulse is generally considered a short pulse when the length is from around 1 to around 10 microseconds. However, there is increasing interest in modifying the short pulse designs to use solid-state transmitters for which long pulse transmissions are more appropriate, so that reliability can be increased while taking advantage of the particular capabilities of the short pulse systems. A pulse is generally considered a long pulse when the pulse length is greater than about 50 microseconds. Long pulse waveforms, as are appropriate to solid-state transmitters for long or medium range, low pulse repetition frequency, surveillance radars, typically are of sufficient length (for example, 100 microseconds or more) that the only waveforms which have sufficient doppler tolerance for the target radial speeds of interest are linear FM or some form of closely related non-linear FM. An exception may be low frequency, for example, VHF or UHF, radars, for which biphase or quadriphase long coded waveforms may have sufficient doppler tolerance. However, existing radars using tube transmitters typically use short coded pulse waveforms, such as the 6.5 microsecond quadriphase coded pulse used in the TPS-70 radar. These existing radars typically employ digital pulse compression by convolutional methods and, as in the TPS-70 case, also frequently take advantage of the waveform code structure to employ hard limiting type constant false alarm rate (CFAR) processing. A conventional approach to signal processing for long pulses, appropriate to solid-state transmitter type waveforms would likely involve major signal processor changes which possibly would include the incorporation of Fast Fourier Transform (FFT) - Inverse Fast Fourier Transform (IFFT) pulse compression producing a short unencoded pulse from the long encoded pulse and cell averaging CFAR into the processor algorithms.